Use of solar energy has been advancing so as to preserve the environment of the earth, and installation of solar cell modules consisting of solar cell elements connecting a plurality of solar cells to roofs and walls of common buildings and houses are in progression. Meanwhile, implementation of solar cells which incorporate semiconductors that are advantageous to enlarging of size, particularly solar cells made of Si (silicon) and the like are rapidly developing. Enhancement of silicon crystal solar cell which contribute to reduction in costs and enhancement in efficiency of photovoltaic power generation system is an issue to be solved in the future.
In order to improve performance and reliability of the silicon crystal solar cells, it is important to analyze diffusion length of a minority carrier and an in-plane defect, and feedback an analysis result to an optimization design of an arrangement of the solar cell elements and to a production process of the solar cell element.
The in-plane defect remarkably decreases output characteristics of the solar cell elements, which causes bad influence in photoelectric conversion efficiency of the solar cell module. As a result, spread of the silicon crystal solar cells are interrupted due to decrease in the photoelectric conversion efficiency and an increase in costs which are caused by the decrease in the photoelectric conversion efficiency. Accordingly, development of a method for evaluating performance of the solar cell that can detect the in-plane defect is in need for.
Meanwhile, so-called EBIC (Electron Beam Induced Current) and LBIC (Laser Beam Induced Current), that is, methods for measuring a current or voltage induced by using an electron beam or laser beam and thereby analyzing diffusion length of minority carriers and defects (grain boundary/transgranular), are widely used as the method for evaluating the performance of the solar cell, for example.
By the EBIC or LBIC, it is possible to measure and evaluate a degree of an elective activity or diffusion length of the minority carriers in solar cells locally. By use of a result of this measurement and evaluation, evaluation of the photoelectric conversion efficiency and quality of the solar cell is possible (see Non Patent Document 1).
Moreover, an apparatus has been revealed, which apparatus analyzes, based on infrared light intensity, distribution of heat generated due to a bias in a forward direction, so as to detect a short circuit section (see Non Patent Document 2).
Furthermore, a technique has also been revealed that a back side of a substrate is exposed to strong light so as to detect leakage of light, which as a result detects a substrate crack (see Non Patent Document 3).
[Non Patent Document 1]
    N. Sakitani, et al., “Evaluation of Recombination Velocity at Grain Boundaries in Poly-Si Solar Cells with Laser Beam Induced Current” Solid State Phenomena Vol. 93 (2003), pp. 351-354[Non Patent Document 2]    J. Isenberg, et al., “SPATIALLY RESOLVED IR-MEASUREMENT TECHNIQUES FOR SOLAR CELLS” Presented at the 19th European Photovoltaic Solar Energy Conference, 7-11 Jun. 2004, Paris[Non Patent Document 3]    Rueland, et al., “OPTICAL μ-CRACK DETECTION IN COMBINATION WITH STABILITY TESTING FOR IN-LINE-INSPECTION OF WAFERS AND CELLS” 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain
The in-plane defects in a solar cell are, more specifically, faults of external causes such as a substrate crack, electrode rupture, loose connection and the like, and faults of internal causes such as crystal defect, dislocation, grain boundaries and the like that are caused by physical properties of the substrate material.
Not only is the substrate crack large in size which occurs so as to cross the substrate, but also generates in minute areas inside the substrate. The substrate crack causes bad influence to photoelectric conversion functions such as reduction of photoproduction current as a centroid of a recombination of minority carriers, and a rise in series resistance by blocking the current passage. Many of the faults of external causes generate due to fragility of mechanic intensity and external force (including thermal warp) given to the substrate during the production process of the solar cell. Simple detection of the defect and feedback of this to the conditions of the production process so as to make improvements links directly to improvement of long-term reliability and increase in production yield rate of the solar cell. Moreover, analysis of whether the cause of the decrease in the photoelectric conversion function is the fault of the external cause mainly based on mechanic intensity or the fault of the internal cause based on material property, connects to high-function and high-reliability of the solar cell. Accordingly, carrying out the detection of these faults easily links to future implementation and spreading of the solar cells.
However, the foregoing EBIC and LBIC devices require a large-scale apparatus to detect the defect of the solar cell. This causes the need for large sum investment for plant and equipment. Further, with the EBIC and LBIC devices, there are many limitations in arrangements such that a two-dimensional scanner having a good position decision accuracy and a scanner probe that uses electron beams or lasers are required to measure a two-dimensional distribution of the minority carrier diffusion length, an electron microscope is required so as to irradiate the electron beams, and a multiwavelength light source is required for radiating a laser beam. As such, a problem exists such that the method for evaluating the solar cell module is not easily carried out.
Moreover, a device which analyzes an exothermic distribution from the solar cell has poor sensitivity and resolving power. Therefore, the defects occurring in the solar cell cannot be accurately detected.
Furthermore, with the technique of detecting the substrate crack by detecting the light leakage, it is not possible to detect minute cracks in which light does not leak, that is, so-called hair cracks, micro cracks and the like.
Therefore, there has been a strong demand for development of (i) a method and (ii) an apparatus for evaluating a solar cell, each of which make it possible to easily and accurately evaluate photoelectric conversion performance of a solar cell module and (iii) use thereof. Especially, silicon polycrystalline solar cell has been rapidly advanced to practical use. Development of an evaluation method and the like which contributes to high performance of the polycrystalline solar cell is an immediate issue, in which various defects in the solar cell are detected.
In view of the aforementioned problem, an object of the present invention is to provide (i) a method and (ii) an apparatus for evaluating a solar cell, each of which makes it possible to easily and accurately evaluate a solar cell module in terms of its photoelectric conversion, without requiring a large-sized facility and (iii) use thereof.